The object of this invention is a compostable polymer coated paper or paperboard, in which the coating includes as one component polylactide. A further object of the invention is a method for producing the paper or paperboard in question as well as a number of products produced from the paper or paperboard.
Water-resistant polymer coated paper and paperboard are commonly used as packaging material for food stuffs and as disposable containers. Coating polymers and multilamellar coating structures including the same have been developed that give to the package a good oxygen, water vapour and aroma tightness in order to ascertain good preservation of the packaged product. Another demand of lately increasing importance set on polymer coated packaging materials in their biodegradability. However, these two objects cannot be reached with the same polymers because the efficient block polymers in use such as EVOH or polyamide are practically unbiodegradable where as in commercial biodegradable polymers the blocking properties have more or less been omitted. Examples of biodegradable polymers on the market are e.g. polyhydroxy butyrate, plastics based on starch and polylactide (PLA) the latter of which is preferred for its simple manufacture and for its relatively good blocking properties.
Use of polylactide as packaging material coating is known e.g. from FI Patent Application 951 637 that describes a grease resistant paper intended for food stuff packaging where polylactide is mentioned as one possible material for the biodegradable polymer coating. According to this publication, the coating on the paper may consist of one biodegradable polymer layer or of a layered structure of two, even three layers on top of one another where the different layers have their own functions. The object was to produce a packaging paper which were at the same time grease, aroma, gas and water vapour proof and biodegradable. However, the publication does not include practical examples of application with measuring data, also, the publication does not divulge the role of the biodegradable polylactide might have in a multilamellar coating.
A more specific description of the use of polylactide in a packaging material may be found in EP Application 0 514 137 which describes a polylactide coated biodegradable packaging paper or paperboard for packaging liquids. The paper or paperboard can first be coated with a biodegradable adhesion producing binder layer that may be glue, gelatine, casein or starch after which the polylactide layer is applied on top of the binder layer. However according to this publication, all paper types do not require use of binder. In the examples given in the publication the thickness of the polylactide layer is 25-30 xcexcm, and the coating is described as transparent, glossy, moisture resistant and as having good mechanical strength. In the examples of application in this publication the biodegradability of the coating has been tested but not its barrier properties.
In the brochure called xe2x80x9cPoly(Lactic Acid) Polymersxe2x80x9d (1977) published by Neste Oy, the manufacturer of polylactide, polylactide in itself is brittle and stiff if plasticizers are not included. The polyractide film forms a good gas barrier for air but only a fair one for water vapour. The flexibility and strength improving plasticizers added may disturb the barrier properties of the polylactide. According to this information, polylactide is no comparison for the best biodegradable blocking polymers (EVOH) but however, as to its gas barrier properties, essentially better than e.g. the polyethylene commonly used and satisfactory for numerous container and packaging applications. According to this brochure polylactide may be extruded and heat sealed at 180-250xc2x0 C.
Accruing to observations and experience of the inventors, when extruding polylactide directly on paper or paperboard one comes up against the problem that in order to attain sufficient adhesion, one must operate at the utmost allowed temperature, i.e. at about 250xc2x0 C., where furthermore the layer applied must be relatively thick. In the temperature mentioned the melt strength of polylactide is weak and pin holes remain easily in the coating layer obtained. Furthermore, because of the high temperature the polylactide is at risk of degradation such that its average molecular weight and viscosity decrease which further increases the fragility of the layer and promotes pin hole formation. Due to the brittleness of polylactide the products produced of a paperboard coated therewith show leaks and cracks whereby the coating does not withstand the creasing or bending and extending according to form inherent in producing plate or mould form products. Even in heat sealing polylactide layers cracking diminishing the tightness of seals is encountered.
Even dilution of coatings is included in developing trends of polymer coated packaging material, whereby e.g. LD polyethylene has been used with good results. As already mentioned, polylactide can be made to adhere to paper or paperboard only as a relatively thick layer, corresponding to what is described in EP Patent Application 0 514 137. With thinner layers the adhesion to the paper or paperboard surface is insufficient where in addition hindering of pin hole formation becomes all the more difficult.
However, decreasing the polylactide content would be desirable even because of the costs involved in using this material.
Still a further significant problem encountered with polylactide coatings is tearing of the edges of the coated film and polymer flakes loosening therefrom which when spilling to the newly applied polymer coating cause irregularities of the coating and clogging of the process. The high processing temperatures of polylactide promote xe2x80x9cbeardingxe2x80x9d of the mouth piece of the extruder due to which the former must be cleansed and production ceased.
The purpose of the invention at hand is to come up with a solution by which the polylactide content in the paper or paperboard polymer coating can be diminished when, at the same time, the other inconveniences encountered with polylactide coatings described above can essentially be avoided. The polymer coated paper or paperboard according to invention is characterized in that the coating comprises an outer layer containing polylactide which weights at most about 20 g/cm3 plus an adhesive layer binding the outer layer to the paper or paperboard which consists of a polylactide co-extrudable biodegradable polymer material.
With the inventive coextrusion of polylactide and the biodegradable polymer functioning as adhesion material on paper or paperboard are avoided the tearing of film occurring when extruding polylactide alone and the concomitant problems as the adhesion polymer acts as compounding element of the film. As the adhesive layer takes care of the problem presented by polylactide adherence the extrusion temperature may be lowered which in turn enhances the quality of the polylactide layer formed and diminishes the susceptibility of the material to tear and form pin holes when at the same time the polylactide layer may be made thinner than before. As a whole, the bilayered biodegradable polymer coating according to invention is more flexible and tighter than previous unilamellar polylactide coatings, in addition to which the amount of polymer in the polylactide layer or even in the coating as a whole can be diminished where by cost savings are produced.
It is preferred that the materials in the adhesive layer according to invention are biodegradable polyesters such as cellulose esters, aliphatic or aliphatic-aromatic co-polyesters or mixtures thereof. As cellulose esters may be mentioned cellulose acetate, propionate, butyrate, acetate-propionate, acetate-butyrate and propionate-butyrate. As copolyesters may be mentioned polyhydroxy alcanolates such as polyhydroxy butyrate, polyhydroxy valerate and polyhydroxy butyrate-polyhydroxy valerate copolymer. The mixtures mentioned are in particular binary or tertiary mixtures of cellulose esters and the copolyesters mentioned. Biodegradable adhesive polymers are described e.g. in U.S. Pat. Nos. 5,446,079; 5,580,911; 5,599,858; and 5,661,193, all of which are included hereby as references. As examples of products commercially useful may be mentioned cellulose acetate ester 14326 and copolyester 14766 manufactured by Eastman Chemical Company. Because of their strong adhesivity these polymers could not be extruded as such as a layer on a paper or paperboard surface where as their coextrusion with polylactide is extremely successful whereby the polylactide forms a surface on the coextruded film that as less adhesive can withstand contact with the rollers guiding the newly coated paper or paperboard conveyor belt. By using copolyesters, it has been possible to drop the extrusion temperature to about 200-240xc2x0 C. which presents an essential advantage in regard not only to hindering the degradation of the polylactide but also as to avoiding the falty taste due to the packaging material of the food stuff. The total weight of the coating produced by coextrusion of copolyester and polylactide may according to preliminary tests be dropped to about 5-15 g/m2 whereby the amount of polymer is of the same order of magnitude as in the previously knowm LD polyethylene coated paper and paperboard.
Even polyester amides such as polyester amides BAK 402-005 and BAK 2195 manufactured by Bayer are preferred biodegradable polymers well suited as components of the adhesive layer. Their adhesivity is that much lower that they can withstand the contact with rollers in the coating process. This makes possible the inventive application in which a trilayered polymer coating is produced on the cardboard that comprises the polylactide layer in between two adhesive layers. The advantages of this solution are presented as a smaller constriction of the coextruded polymer film, the ever lessened susceptibility of the coating to form pin holes and the improved heat sealability of the coated paperboard especially when adhering the polymer surface to the paperboard surface. The latter is caused by the polyester amide""s stronger melt strength than that of the polylactide whereby it does not as easily penetrate the paperboard but remains at the intervening surface, forming an intact and durable seam.
Further, it is possible to enhance the aroma, oxygen and/or water vapour barring properties of the coated paperboard by introducing a biodegradable barrier polymer layer between the polylactide layer and the adhesive layer, such as PVA (polyvinyl alcohol) or PVA copolymer layer. As a result, one gets a trilayered structure composed of the polylactide, barrier polymer and the adhesive polymer, or a four-layered structure when even the polylactide and the barrier polymer require in order to attach an adhesive layer in between. In each and every case the coating may according to invention be formed by coextrusion of the layers.
According to one preferred embodiment of the invention, the compostable coating includes mixed in the polymer a finely divided mineral component. With a suitable mineral the barring properties of the polylactide layer can be profitably affected, in addition to which the mineral particles as substitutes for the polymer may speed up biological break-down of the layer at the dumping site.
In particular, it is possible to precoat the paperboard with a polymer latex containing dispersed therein mineral particles whereby coextrusion of the adhesive polymer layer and polylactide layer is conducted on a dispersion precoated paperboard.
Depending on the intended use of the paper or paperboard, it may be provided with a polymer coating only on one side, or on both sides. The paperboard coated according to invention that is well suited for food stuff packaging and for disposable dishes or containers is preferably a trilayered paperboard whereby in the middle there is a thicker layer of a mixture of chemical mass and CTMNP on both sides of which are arranged thinner layers composed essentially of pure chemical mass.
The inventive method for producing compostable polymer coated paper or paperboard is characterized in that the outer layer containing polylactide and the adhesive layer composed of biodegradable polymer material are coextruded together on a moving paper or paperboard conveyor belt. As to the applications of the extrudable layers, we refer to the above description of the inventive paper or paperboard.
As products according to the invention that are formed of the coated paperboard described above may be mentioned especially: containers for frozen food, drinking cups or goblets and heat-sealed cartons that are packaged with essentially liquid food stuffs. In cups intended for hot drinks it is enough to have the water tight coating only on the inner surface of the cup. Contrary to this, in cups intended for cold drinks it is preferred that the coating be on both the inner and the outer surface of the cup whereby the carton is protected even from water condensing on the outer surface of the cup. In heat sealed cartons, at least the inner surface of the carton must be coated even if as to the heat sealability it is preferred that the sealable polymer coating be on both the inner and the outer surface of the container.
The inventive polymer coated paper is especially well suited as wrapping material for food stuffs. The polymer coating is most preferably on only the inner surface of this paper that touches the food stuff.